Efficient Singlet Oxygen Generation from Polymers Derivatized with Hexanuclear Molybdenum Clusters

The molybdenum(II) chloride cluster Mo6Cl12 has been immobilized on 2% cross-linked poly(4-vinylpyridine) (PVP-Mo6Cl12) by the axial ligation of two pendant pyridines of the polymer to the hexanuclear metal core. Polymers with different cluster loadings exhibit the characteristic broad band emission of Mo6Cl12 at λmax = 765 ± 3 nm, and luminescence decays are composed of short- and long-lifetime components, which are consistent with the presence of cis- and trans-disubstituted cluster species, respectively. The lifetime of the cis form is too short to permit its bimolecular reaction with oxygen. However, the long-lifetime component is quenched efficiently by O2, and Stern−Volmer analyses of emission lifetimes yield quenching rate constants of (2−10) × 105 M-1 s-1 for cluster loadings in the range (3.5−0.3) × 10-3 g/g of PVP, respectively. The quenching rate constant increases to ∼50 × 105 M-1 s-1 for all polymers in MeOH regardless of cluster loading. In aqueous solutions (pH 3−6) intermediate quenching r...